Adding or removing a Plex Live TV favorite channel never persisted: the PUT
to epg.provider.plex.tv/settings/favoriteChannels answered 400. The dio ->
package:http migration (15b22f75) lost dio's implicit JSON content type, so
the JSON-encoded favorites list went out as text/plain and the Plex cloud
refused to parse the body. Verified live: the identical payload succeeds
with application/json and the mutation persists.
MediaServerHttpClient now defaults content-type to application/json for
structured bodies. Callers and client defaults still win (the headers map
is case-insensitive and already populated), so Jellyfin's pinned default
and the octet-stream artwork upload are unaffected; favorites is the only
Plex request with a JSON body.
close#1878
edde746
bb642fce03